With reference to FIG. 1, a system for the offshore mooring of a buoyant structure 1 (for example such as a drilling platform, vessel, wind turbine, etc.) generally comprises at least one mooring chain 2 (or anchoring line) acting as a connection between said buoyant structure and an anchoring point 3 in the sea bed. In practice, the buoyant structure 1 is provided with a multitude of mooring chains 2. The latter are essential for holding the structure 1 in position and for the proper operation of the production process which should be relatively unaffected by the movements of said structure induced by winds, waves, tides or currents.
When the mooring chain 2 is connected to the anchoring point 3, a hauling device 4 fitted on the buoyant structure 1, makes it possible to tension said chain. In practice, the value of this pretension of the chain 2 corresponds to a predefined percentage of the rupture load of said chain. When this value is reached, the chain 2 is locked in position by means of a chain stopping device 5, commonly referred to using the term “Chain Stopper”.
In a typical installation, the chain stopping device 5 is installed above the hauling device 4. One or a plurality of pulleys 10 may be provided between the chain stopper 5 and the hauling device 4. With reference to FIG. 2, the chain stopper 5 generally comprises a locking housing 50 provided with a channel 53 inside which the chain 2 is capable of sliding. This housing 50 is connected to the buoyant structure 1 by connection means 51 (shown in FIG. 1), such as a yoke, ball joint or any other hinge system, configured to direct the axis of said channel to the pulling axis A of the chain 2. In this way, the latter systematically applies a tensile or not flexion load. To lock the chain 2 in position, the housing 50 has built-in bolts 52 inhibiting the sliding of said chain in said channel. As a general rule, these bolts 52 are engaged with a link of the chain 2 to hold the link in position. The locking of the chain 2 is in principle unidirectional, the bolts 52 merely preventing the portion of chain situated between the hauling device 6 and the chain stopper 5 from entering the housing 50.
Given that the positioning of the mooring chains 2 is complex, it is important to ensure that they do not break under the effect of excessive strain due to the movements of the buoyant structure 1 induced by winds, waves, tides or currents. The rupture of a mooring chain is liable to destabilize the buoyant structure 1, damage drilling tools or pumping lines and thus shut down production. To prevent this, chain stopping devices generally comprise means for measuring the tension in the mooring chain. It is preferable for these means to measure the tension in the pulling axis of the mooring chain, but without being in direct contact with said chain to prevent any excessive load.
In the patent document EP 0.966.396 (BARDEX ENGINEERING), the measurement means consist of an extensometer associated with the locking housing. More specifically, this extensometer is secured to the outer wall of the housing, in the axis of the channel, i.e. in the pulling axis of the chain. This technical solution is not fully satisfactory. Indeed, the extensometer is in direct contact with the external environment, and particularly the sea and spray, such that particular attention should be paid to the tightness thereof to prevent damage to the electronic components. A further drawback lies in the fact that a sudden movement of the mooring chain is merely needed for the locking housing to abut violently against the buoyant structure and be damaged. A further drawback is associated with the need to re-calibrate the extensometer regularly.
In the patent document US 2010/0175604 (BOATMAN), the measurement means consist of:—either a strain gauge positioned on the locking housing;—or a measurement cell positioned between the bolts and a guide whereon said bolts rest; or a contactless sensor positioned at a trunnion block. The use of a strain gauge or a contactless sensor gives rise to the same disadvantages as those cited in the previous paragraph. The measurement cell is of the “standard” type, and for example described in the patent documents GB 2475081 (ILLINOIS TOOL WORKS) or CA2019143 (MCKENNA).
In the patent document US 2003/0155564 (FONTENOT), the measurement means consist of an electronic sensor or a contactless sensor which also gives rise to reliability and calibration problems.
The patent document U.S. Pat. No. 3,613,517 (BRADLEY) discloses a cell for measuring tensile forces. This cell is presented in the form of an annular piston comprising a cylindrical element mounted telescopically in a basic element. The space between these two elements is filled with fluid. When a force is applied on the movable element, the fluid contained in the space is compressed. The pressure measurement of this fluid makes it possible to deduce the intensity of the force applied on the movable element. This type of cell is complex to produce and involves dealing with tightness problems, particularly using a flexible membrane. In practice, this membrane is particularly fragile and cannot be placed in direct contact with the sea or spray, as salt is liable to damage said membrane irreversibly. The BRADLEY cell is thus unsuitable for measuring the tension of a mooring chain.